In general, an engine obtains power by combustion of fuel, fuel mixed with air is combusted in a combustion chamber of the engine, and gas produced by combustion in the combustion chamber is passed to an exhaust tube through an exhaust manifold and then discharged to the outside. As a temperature of exhaust gas in the engine is increased due to strict regulations on exhaust gas, the exhaust manifold has a structure that is divided into three portions in order to prevent cracks or breaks thereof. For example, in a case of a six-cylinder engine, the exhaust manifold is manufactured to be divided into three pieces each of which corresponds to two cylinders, and a single completed exhaust manifold is used by assembling the three pieces.
FIG. 1 is a schematic perspective view illustrating an exhaust manifold of an engine according to an example of the related art.
As illustrated in FIG. 1, an exhaust manifold 1 of an engine according to an example of the related art includes a hollow-shaped main tube 2, branch tubes 3 which are provided on one side of the main tube at predetermined intervals, respectively and have exhaust gas intake ports 3a, and an exhaust gas discharge port 2a which is provided on the other side of the main tube 2 and communicated with each of the branch tubes 3 through the main tube.
In addition, at an outer side periphery of the exhaust manifold 1, there is provided a plurality of bosses 4 having a threaded hole at a center thereof, and each of the bosses may detachably fix peripheral structures such as various types of pipes and the like by means of bolts and the like.
Therefore, when the engine is driven, exhaust gas combusted in each of the combustion chambers flows into the exhaust gas intake port 3a of the corresponding branch tube 3, and then may be discharged to an exhaust tube (not illustrated) through the exhaust gas discharge port 2a via the main tube 2.
Meanwhile, FIG. 2 is a schematic perspective view illustrating a state in which one side connection portion positioned at a central side of the exhaust manifold of the engine according to the related art and the other side corresponding connection portion to be coupled to the one side connection portion are separated from each other, and FIG. 3 is a schematic cross-sectional view illustrating a state in which a connection portion of the exhaust manifold of the engine according to the related art and a corresponding connection portion are coupled to each other.
As illustrated in FIG. 2, each of the connection portions 5 and 5A of the exhaust manifold 1 of the engine according to the related art has two sealing grooves 5a formed along a length thereof at a predetermined interval, and one groove 5b for blocking exhaust gas leakage is formed between the sealing grooves 5a. Two seal rings 6 are configured as one set, and detachably fitted into the respective sealing grooves 5a. 
However, in order to prevent physical interference between a front end portion of one side connection portion 5 and a corresponding portion of the other side corresponding connection portion 5A, the exhaust manifold 1 of the engine according to the related art is formed to have a shape in which a predetermined marginal space is merely provided in a longitudinal direction (see FIG. 3). For this reason, the exhaust manifold 1 of the engine according to the related art has a structure in which when the engine is driven, high exhaust pressure is inevitably applied to the connection portions through the marginal space.
That is, in the exhaust manifold 1 of the engine according to the related art, because one side connection portion 5 and the other side corresponding connection portion 5A are not yet sufficiently and thermally expanded when the engine starts to be driven in a cold state, a gap between the seal rings 6 of one side connection portion 5 and the other side corresponding connection portion 5A is large, and for this reason, there is a problem in that condensate, which is produced in the exhaust manifold 1 by contact with the outside air through the gap is discharged to the outside through the gap by high exhaust pressure.
In addition, in the exhaust manifold 1 of the engine of the related art, there is a problem in that when high exhaust pressure is generated as the engine starts to be driven in a cold state as described above, not only condensate but also exhaust gas leaks to the atmosphere through the gap.
Thus, since condensate and/or exhaust gas leak to the outside peripheries of the engine through the gap between the connection portions 5 and 5A of the exhaust manifold 1, and the peripheries of the engine are contaminated, there is a number of complaints from customers about the above problem, it is cumbersome to perform maintenance, and there is a burden that the relevant components need to be replaced in a severe case.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.